It is very rare that the temperature at which your t-stat is set is equal the temp reading in your tub.
You need to adjust you t-stat until the hot spot reaches the right temp.
The t-stat probe should be on the heat source, not in the enclosure.
The thermometer probe should be above the hot spot, I would however recommend a temp gun instead.
The difference between the t--stat temp and hot spot temps will depends on various things, heat pad, thickness of enclosure, substrate etc.
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